Anytime a person chooses to have sexual intercourse with another person for the sole purpose of getting that person through an awkward or sad moment. Typically there is no actual interest romantically in that person, but the idea of engaging in intercourse is not horrifying and almost a bit sporty. The giver goes through the motions of actual romance without any actual feelings.

Sally felt bad for Jim during his visit after the loss of his girlfriend several weeks earlier, and while nothing she tried that afternoon could snap him out of his sadness, she decided to give him a pity-fuck which would show him that happiness could be found again.

To have sex with someone for reasons stemming from pity rather than desire. The phrase allows a range of implications from concern about the unfulfilment of the fuckee or feelings of superiority over them.

Kate came round last night and looked so dejected, I had to pity-fuck her.

noun, what you give a female who might have trouble getting someone to fuck her.

Denise is my favorite pity fuck on a weekend, I take her sailing, don't have to show her off, she cooks & cleans & washes up, I fuck her in the dark; she's not coyote ugly but close to it. Not someone I want to be seen with.